Silicon-based gunpowder may propel MEMS devices

SAN DIEGO  The accidental discovery of a silicon form of "gunpowder" is pointing the way toward integrating nanoscale explosives onto silicon chips, a technology that might provide microelectromechanical systems (MEMS) with a means for rocket propulsion.

Chemists at the University of California, San Diego, had a chip blow up in their faces after scratching a porous silicon wafer that had been impregnated with gadolinium nitrate. The formula is now being proposed as a means to create self-destruct mechanisms, on-chip retro rockets or fingernail-size chemical analyzers.

"It's about the equivalent-size explosion you would get from a couple of caps in a kid's toy gun," said researcher Michael Sailor. "We could use it to give MEMS robots a means to move around  they could just shoot a little retro rocket to flip over or jump a few inches and go analyze another part of their territory." The technology could also, he said, "provide a flame source for handheld chemistry labs."

Frederic Mikulec, a postdoctoral researcher, and Joseph Kirtland, a student from Union College in Schenectady, N.Y., both working in Sailor's lab, also contributed to the study, which was financed by the National Science Foundation and the Tactical Sensors Program at the Defense Advanced Research Projects Agency (Darpa).

While the researchers were not systematically searching for a silicon version of gunpowder, they have been engaged in a comprehensive program to develop porous silicon as a sensing medium for explosives and toxic gases that might be used by terrorists. Essentially, porous silicon forms as a random array of nanometer-diameter silicon wires. When treated with different chemicals, the wires alter their electrical properties when in contact with a few molecules of a target substance.

Last year Sailor, working with William Trogler, another UCSD chemist, devised a silicon wire-polymer combination that was able to detect tiny amounts of TNT. The wire consisted of a string of silicon atoms encased in organic molecules that are photoluminescent.

A current passing through the wire makes it glow. Only a few molecules of TNT will stop the current, so that the presence of the explosive will quench the emitted light, providing a detection signal.

Porous silicon provides a handy source of silicon nanowires and Sailor's lab has been busy saturating porous-silicon wafers with various dopants in the quest for practical and robust chemical sensors. Porous silicon can also be fabricated in multilayers so that a doped layer can change its optical properties  in the presence of target chemicals  relative to another layer to provide highly sensitive chemical detection. One result of the work is a handheld sensor that can detect extremely small quantities of the nerve gas sarin.

Explosive discovery

The scientists were on the trail of another type of chemical sensor when they baked gadolinium nitrate into the nanopores of porous silicon. Only later did they realize they had created a formula similar to gunpowder, but using silicon as a base instead of carbon.

The UCSD team already knew that, like gunpowder, a silicon-based material would explode when mixed with potassium nitrate. The gadolinium nitrate formula was purely accidental.

"A charcoal briquette will burn slowly, but if you grind it up into a fine powder and mix it with the right chemicals it becomes gunpowder," explained Sailor. "We did the same trick with silicon. We prepared the silicon in a form that is very highly divided  a nanocrystal  with a process that makes it so small and with such a high surface area that it becomes very reactive."

The silicon becomes like a sponge with nanometer-size holes. When filled with a nitrate salt the resulting system behaves chemically just like gunpowder, which is finely ground carbon mixed with potassium nitrate and sulfur.

The first application Sailor thought of was a propulsion system for MEMS devices. Such systems have been a difficult thing to miniaturize. According to Sailor, it is easy to make a hand-size rocket engine, but it has proven tough to go much smaller. And to make such tiny rocket engines for the gnat-size mechanisms that can be sculpted from silicon in this very small nanoscale world has so far been impossible.

"Just like gunpowder, you make it up beforehand and stick it in the bullet, but it doesn't go off until you tell it to," Sailor said. "The difference between this little nanoexplosive and a bullet is that you don't need a little hammer hitting it to get it to go off. We can use a little spark of electricity."

Tiny rockets

MEMS devices have plenty of electricity available to generate this spark, he said. "We just have a little spark that flies through the chip when we throw a switch, and that small amount of electricity causes the thing to detonate," Sailor explained. The concept is similar to solid-fuel boosters on rockets. In a MEMS application, the researchers envision an array of thousands of such tiny solid-fuel rocket engines that can be fired off individually by electronics.

Darpa has already begun to look into using the tiny retro rockets to provide propulsion for a MEMS system it is developing for the intelligent battlefield of the future. Already on the drawing board are very small sensors that can be dropped en masse from airplanes to keep tabs on battlefield areas, providing real-time analysis to monitor chemical and biological weapons deployment. However, until now Darpa has viewed these nanoscale outposts as being stationary. Now they can move around.

"For remote surveillance you might want to sprinkle smart dust over a field that you think might be downwind of a chemical-munitions plant behind enemy lines or a biological-warfare plant, and have it hop around," said Sailor. "When it finds something, it sends a signal back up to a satellite by setting off a tiny flare."

Staring into the flame

The inherent purity of the gadolinium- and silicon-based explosive also makes it ideal for use in real-time chemical-analysis equipment that could test for toxic elements remotely in the field. A common laboratory test method is to burn a sample of the material and then analyze the color of the flame, which indicates elements present. Sailor's tiny explosions could provide a flame source for handheld chemistry labs.

"One of the applications we envision for this nanomaterial is to place it outside in a field somewhere to soak up some water coming out of, say, a reservoir," Sailor said. "A very small amount is needed  only microliters, less than a teardrop. Then you ignite it in what is called a flame test; the color of the flame is characteristic of the elements in the sample. You just take a snapshot of the flame and analyze it with a spectrometer."

Other futuristic applications include self-destructing computer chips that destroy themselves when people try to use them for unscrupulous purposes. Such chips "can't be reverse-engineered," Sailor said. "Or you could have your cell phone self-destruct if somebody was using it that wasn't authorized."

Sailor's research group will not be developing any applications for the technology, but it has published its results so that engineers worldwide can employ the process in their own applications.

"We know how to put millions of transistors on a chip, and we know how to make nanoscale gears and plungers and other mechanisms, but there are lots of kinds of tools that we can't just carve out of silicon," said Sailor. "This is another tool in the nanoengineer's tool kit, for engineers who are developing really small devices  a tool for them to do things we know how to do at a larger scale, but until now have not really had the tools to do at the nanoscale."

An audio recording of reporter R. Colin Johnson's full interview with Michael Sailor can be found online at AmpCast.com/RColinJohnson.